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We review the results of recent neutron-scattering studies of the spin fluctuations in samples of La 1.89 Sr 0.11 CuO 4 which are ∼80% superconducting with T c = 10 K. The structure factor, S(Q), reflects three-dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasi-elastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

La 1.9 Ba 0.1 CuO 4 has been observed to undergo the following sequence of transformations upon cooling from 300 K: tetragonal (I4/mmm) equation orthorhombic (Cmca) equation tetragonal (P4 2 /ncm). The newly discovered low-temperature tetragonal structure can be understood geometrically as arising from a coherent superposition of the two domain modifications of the orthorhombic structure. Dynamically, it results from a second instability in the twofold degenerate soft modes of the high-temperature tetragonal phase. Energetically, the system can be modeled as an XY-spin system with temperature-dependent quartic anisotropy ν(T), and the low-temperature transformation coincides with an isotropic point ν(T 1 ) = 0. The relationship of the newly discovered transformation to other anomalous properties and to superconductivity is discussed briefly.